In order to solve the tightness of frequency resources with recent increase in the volume of data communication, new radio frequencies have been allocated for mobile communications so as to proceed with construction of new mobile communication systems(e.g., IMT-Advanced system) using the allocated frequency bands.
For such new mobile communication systems, higher frequency bands compared to the frequency bands that have been allocated to the existing systems will be allocated. However, a signal having the higher frequency will attenuate more greatly so that the coverage will become narrower compared to the existing systems. As a means for solving this problem, there is a method in which a relay station apparatus that relays communication between the base station apparatus and a terminal apparatus is provided in the cell, as shown in FIG. 15.
FIG. 15 shows a conventional radio communication system using a relay station apparatus. Terminal apparatuses 80, 82 and 84 exist in the communication area of a base station apparatus 90. Also, a relay station apparatus 92 is deployed therein. Relay station apparatus 92 re-transmits a signal received from base station apparatus 90 to terminal apparatuses.
Here, the relay station apparatus may be of a type that performs transmission by just amplifying the received signal (Amplify-and-Forward: AF type), a type that performs transmission by demodulating the signal once and re-modulating the signal if no error is found (Decode-and-Forward: DF type) or the like, and performs communication between the base station apparatus and a terminal apparatus far away from the base station apparatus (terminal apparatus that is located near the cell edge) through the relay station itself so as to keep the cellular coverage equivalent to that of the existing system without degrading reception performance of the terminal apparatuses.
However, in the system including this relay station apparatus, when a signal is transmitted to a terminal apparatus that is located a certain distance away from the base station apparatus, the signal for one frame is transmitted using two frames due to the intermediary of the relay station apparatus. Specifically, as shown in FIG. 16, two frames, one frame (frame F900) for transmission from base station apparatus 90 to relay station apparatus 92 and another frame (frame F902) for transmission from relay station apparatus 92 to terminal apparatus 80, are used. Accordingly, there occurs the problem of transmission efficiency being lowered. Here, it should be noted that frame F900 and frame F902 are of the same signal.
As a measure to deal with the above problem, there is a known technology in which a technique called superposition coding is applied to the relay system (e.g., see non-patent document 1).
In non-patent document 1, since the transmission takes a time of two-frames, though reception performance can be improved by increasing each of the numbers of reception and transmissions each by one by use of a relay station apparatus, description is made on how to relay data corresponding to two frames by two frames without reducing frame efficiency.
The radio communication system described in non-patent document 1 will be explained with reference to FIGS. 17 and 18. As shown in FIG. 18, base station apparatus 90 transmits combination of two signals (A and B) that are made different in power, in the first frame F920. Here, a (0<a<0.5) given in FIGS. 17 and 18 is a coefficient for dividing the power for each signal. Since relay station apparatus 92 needs to receive the signal transmitted from base station apparatus 90, the relay station apparatus does not perform transmission in frame F922.
Further, though transmission from base station apparatus 90 to terminal apparatus 82 is usually performed in frame F924, no particular description on this transmission is given in non-patent document 1 . Here, the signal transmitted in frame F922 shown in FIG. 18 is received by relay station apparatus 92 as a received signal r given as the following mathematical expression 1:r=(√{square root over (1−a)}A+√{square root over (a)}B)+n   [Math 1]
where A and B represent modulated signals to be transmitted and n represents noise. Here, no channel variation is considered, for simplicity. The relay station apparatus 92, which has received the signal shown in Math Ex. 1, demodulates signal A that has been transmitted at the higher power, first. At this time, signal B is handled as interference. Then, the demodulated signal is re-modulated, and a subtraction as follows is performed:
                                                        r              =                            ⁢                                                (                                                                                                              1                          -                          a                                                                    ⁢                      A                                        +                                                                  a                                            ⁢                      B                                                        )                                +                n                -                                                                            1                      -                      a                                                        ⁢                  A                                                                                                        =                            ⁢                                                                    a                                    ⁢                  B                                +                n                                                                        [                  Math          ⁢                                          ⁢          2                ]            
When a subtraction of signal A that has been re-modulated by considering power distribution is carried out, signal B added with noise as shown in Math Ex. 2 is extracted. Relay station apparatus 92 demodulates this signal so as to obtain the demodulated result of signal B.
Here, it should be assumed that transmission performance between base station apparatus 90 and relay station apparatus 92 are constantly favorable and the SNR of signal B is high to a certain extent. With this subtraction process, it is also possible to demodulate the signal B that was transmitted at the low power. This relay station apparatus 92 re-transmits the signal B that was transmitted from base station apparatus 90 at the lower power, of the two previously demodulated signals.
Further, in non-patent document 1, the relay station apparatus uses a method by which a signal B′ that is encoded using an encoding rate being different from that of the base station apparatus is re-transmitted, taking into consideration the channel condition between the relay station apparatus and the terminal apparatus. In the terminal apparatus, the signal that was transmitted in the first frame is retained without demodulating and the signal B′ that was transmitted in the next frame is demodulated first. Then, the result is encoded at the encoding rate used in the base station apparatus, then modulated, and multiplied by the coefficient that represents power distribution, so that the calculated result is subtracted from the retained, received signal in the first frame, thereby extracting the signal A, which was transmitted at the higher power from the base station apparatus, for demodudulation. The subtraction process at this point is given as follows.
                                                        r              =                            ⁢                                                (                                                                                                              1                          -                          a                                                                    ⁢                      A                                        +                                                                  a                                            ⁢                      B                                                        )                                +                n                -                                                      a                                    ⁢                  B                                                                                                        =                            ⁢                                                                                          1                      -                      a                                                        ⁢                  A                                +                n                                                                        [                  Math          ⁢                                          ⁢          3                ]            
The above configuration makes it possible to transmit signals for two frames (two signals A and B that are made different in power) in two frames (to transmit signals for three frames in two frame period if the signal of frame 1022 is included), hence enabling transmission via the relay station apparatus without reduction in transmission efficiency.    Non-patent document 1: P. Popovski, E. Carvalho, “Spectrally-Efficient Wireless Relaying based on Superposition Coding, “VTC2007-Spring, April 2007.